OvmfPkg/VirtioFsDxe: add a scatter-gather list data type
    
    In preparation for the variously structured FUSE request/response
    exchanges that virtio-fs uses, introduce a scatter-gather list data type.
    This will let us express FUSE request-response pairs flexibly.
    
    Add a function for validating whether a (request buffer list, response
    buffer list) pair is well-formed, and supported by the Virtio Filesystem
    device's queue depth.
    
    Add another function for mapping and submitting a validated pair of
    scatter-gather lists to the Virtio Filesystem device.
    
    Cc: Ard Biesheuvel <ard.biesheuvel@arm.com>
    Cc: Jordan Justen <jordan.l.justen@intel.com>
    Cc: Philippe Mathieu-Daudé <philmd@redhat.com>
    Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3097
    Signed-off-by: Laszlo Ersek <lersek@redhat.com>